The invention relates to methods and apparatus for performing a ground vibration test in airplanes.
As one important safety regulation of air traffic controllers, all airplane prototypes must pass a so-called “ground vibration test” (“GVT”). In this “ground vibration test”, which will henceforth be referred to as “ground vibration test” in the specification, the airplane or entire airplane structure standing on the ground is made to oscillate by means of exciters (so-called “exciters” or “(vibration) oscillators”). These vibration exciters exert an electromagnetic effect in particular, and are preferably attached in the area of the two wing ends, the two elevator unit ends, the rudder unit end, the aft fuselage, the nose and the engines of the airplane. The vibration exciters are able to excite the structure of the airplane to between 0 Hz and 50 Hz. The vibration amplitude of the vibrations in the fuselage structure generated in this way rarely reaches amplitudes exceeding 5 mm.
The object of the ground vibration test is to determine the vibration form and structural attenuation of the airplane in the area of the measuring transducers in the 0 Hz to 50 Hz frequency range of relevance in terms of the physics of flight. The levels of excitation produced by the electromagnetic vibration exciters are here chosen in such a way as to still be able to readily measure the affected variables and preclude mechanical damage to the integrity of the fuselage structure of the airplane.
In order to acquire the vibrations triggered in the airplane structure by the vibration exciters via measurement, up to 1,000 measuring transducers, preferably accelerometers, are arranged on the airplane for a ground vibration test.
Prior to the ground vibration test, a strictly computational vibration model of the airplane or airplane structure is generated. This theoretical dynamic vibration model of the airplane can encompass up to 10 million interacting node points, each with six degrees of freedom (three translatory and three rotational movements). The acceleration values and/or other measured values ascertained in real time during the ground vibration test are used to adjust or fine-tune the theoretical dynamic model of the airplane determined beforehand as required.
In the method known from prior art, the airplane rests on the ground during the ground vibration test, for example on its landing gear tires. In order to better decouple the airplane structure excited to undergo vibrations and the ground, the air pressure in the tires is generally reduced. It is further known to catch the landing gear of the airplane in a plurality of rubber rings or store the landing gear on air cushions on the hangar floor. The nonlinear tire damping effects must be canceled out of the determined measured values by means of complex algorithms.